In U.S. Pat. No. 5,226,206, there is described an electric toothbrush having twin brush heads which are drivingly connected together by a rack and pinion assembly in which the rack is reciprocated via a drive shaft connected to a crank shaft mounted at one end for rotation on a gear driven by a motor. The twin brush heads are therefore able to oscillate about their respective axes either in the same direction or in opposite directions, depending upon the configuration of the rack, this being generally ‘U’ shaped so as to facilitate the use of gear teeth on the inside thereof for driving the respective heads via respective pinions in mesh therewith.
A potential problem with this arrangement is that it is relatively complex and is not easily cleanable with running water such that the moving parts may become clogged with toothpaste or debris. In addition, the complexity and number of the parts used to provide the oscillating motion for the twin heads adds to the cost and complexity of manufacture.
In U.S. Pat. No. 5,353,460 the use of a rack and pinion assembly is dispensed with in favour of a crank mechanism connecting the first brush head to the second brush head. The first brush head is directly driven by a miter gear or crown wheel which then, in turn, drives the second brush head to oscillate in the opposite direction to that of the first brush head. This arrangement involves fewer moving parts but still depends on the presence of gears and the crank mechanism for its operation, leading to the possibility of these moving parts being difficult to clean and becoming clogged etc. In addition, because the crank arm necessarily has to connect the respective brush heads on the sides thereof remote from the bristles, it will be understood that additional space is required to provide for free movement of the crank arm when the brush is in use. This also has the disadvantage in that the working ends of the crank linkage can never be directly exposed to running water for rinsing and cleaning purposes.
In a first aspect the present invention is derived from the realization that it would be preferable to have an electric toothbrush having multiple, such as two, rotatable brush heads but which dispenses with the need for gears or crank mechanisms for transferring oscillating rotary motion from one brush to another, typically a first brush driven by a connection to an electric motor and the second brush driven by movement of the first brush.